Precipitation and water on the tracks can take a toll. Water can cause the electrical insulators to arc, sending up sparks that are capable of smoldering on nearby equipment. But there are other reasons for smoke on the tracks, including faulty train brakes.

Smoke turned deadly on a Metro train in January, when Virginia commuter Carol Glover died inside a train trapped on the Yellow Line near the L'Enfant Plaza Station. An investigation found that fans meant to clear smoke from the station actually pulled more smoke into the train, and Glover died of acute respiratory failure due to smoke exposure. Nearly 70 other people were taken to hospitals.

Published at 5:55 PM EDT on May 11, 2015 | Updated at 6:32 PM EDT on May 11, 2015